Tetrabromobisphenol S-induced developmental toxicity in zebrafish embryos: impacts on cytoskeletal proteins during gastrulation.

3,3',5,5'-Tetrabromobisphenol S (TBBPS) is an emerging brominated flame retardant that is widely detected in the environment, yet its toxic effects remain poorly understood. The objective of this study was to use zebrafish as a model and determine the effects of TBBPS exposure on gastrulation stages of embryogenesis. We initiated TBBPS exposures (0 or 40 µM) at 0.75 h post fertilization (hpf), phenotyped hourly through cleavage, blastula, and gastrula stages, and showed that TBBPS-treated embryos exhibited delay in development beginning at ∼4 hpf, a stage corresponding to early gastrulation, with significant mortality during late gastrulation. To examine the genetic basis of TBBPS-induced effects, we conducted mRNA sequencing on exposures from 0.75 to 7 hpf, revealing downregulation of cytoskeletal organization and tight junction assembly. We then fluorescent-stained for multiple cytoskeletal and tight junction proteins (α-tubulin, β-tubulin, Zonula occludens-1, filamentous actin [F-actin]) following exposures to a wide range of TBBPS concentrations (0, 0.004, 0.04, 0.4, 4, and 40 µM). F-actin expression was consistently upregulated across all exposed TBBPS concentrations. To determine whether activation of p38, a key regulator of F-actin polymerization, plays a role in TBBPS-induced F-actin disruption, we co-exposed embryos with TBBPS (0 or 40 µM) and a known p38 inhibitor (5 µM SB 203580) and assessed developmental phenotypes and F-actin levels. Treatment with p38 inhibitor significantly rescued TBBPS-induced effects, with co-exposure groups exhibiting cell stages and F-actin expression similar to controls. Collectively, these findings demonstrate that TBBPS exposures target gastrulation stages and define a novel mechanism in which TBBPS increases F-actin polymerization through p38/mitogen-activated protein kinase signaling pathways.